It's hard to review Killer7 by conventional standards when it is a very unconventional game.
So why would anyone bother with this game? Well, it's unique, and it has a very unusual story, in which you are never quite sure what is going on. The characters throughout speak in riddles in very intimidating whispering voices, to which you must find solutions to a puzzle in order to proceed.
In the end, if you are an experienced player and wish to try something very different that defies description, than by all means, purchase Killer7. It's very story driven without feeling forced, and it's a game that you'll get a kick out of showing it off to your friends on just how weird it is.